A rapidly growing technique being employed by many molecular biology laboratories is the amplification of nucleic acid sequences. This is accomplished by means of a polymerase chain reaction (PCR) technique which may be carried out, for example, by thermal cycling instruments such as those of Perkin-Elmer Cetus. A typical instrument for automating PCR comprises a temperature-controlled sample block having a plurality of wells. A block may have, for example, 96 such wells in an 8.times.12 format. Each well receives a thin-walled reaction tube having a sample volume of 5-100 microliters. Nucleic acids and reagents are placed within these tubes which are then placed into wells in a temperature-controlled sample block. The system is then cycled through a heating and cooling sequence for achieving the desired DNA amplification.
It is important for a PCR researcher to be able to verify the accuracy, reproducibility, and uniformity of the temperatures in a PCR instrument. This is difficult to accomplish. For example, a glass liquid thermometer can measure temperatures accurately to .+-.0.03.degree. C. when fully immersed. However, if only the bulb end is placed in a sample well, the accuracy of measurement is greatly reduced. Accuracy is even further eroded if the thermometer is not held in the same position for each measurement.
Thermocouple devices are also subject to inaccuracies. Their accuracy depends upon the position of the thermocouple, the thickness of the tube walls, the accuracy of fit of the tube, and the heat conductance of the thermocouple wire.